Millions of people visit Florence each year to admire its cultural treasures. One of them is the world-famous Bargello Museum, home to great works by Michelangelo and Donatello. Up until the 18th century, this was a grim prison, where executions were carried out on the very spot where I'm standing.

The truth is that the history of Florence, as glorious as it is, is also soaked in blood. From religious wars, to the intrigues and assassinations of the powerful Medici family, to the modern-day story we're going to tell you tonight. It's about a killer unlike any other. It’s about a killer who is, in some ways, still claiming victims today.

Doug Preston: I'm a crime novelist. I write thrillers. And I've never encountered a story like this either in fiction or in non-fiction. It's really a story of tremendous evil.

Italy has seen its share of murder over the centuries, but it was never like this. It’s a case that's been going on for decades, about a series of crimes so gruesome and so incomprehensible that seasoned investigators came to believe the devil himself was behind them.

It was an investigation like no other -– one in which the hunters became the hunted.

Preston: I feel like I've fallen into one of my novels. Because now I'm under investigation.

For best-selling author Douglas Preston, it all began innocently enough six years ago when he decided to fulfill a longtime dream and write a novel set in Italy.

Preston: We'll rent a villa in the country outside of Florence, with olive trees and cypresses around us. Overlooking a vineyard. And isn't it going to be wonderful?

They made their home in the hills of Tuscany in a gorgeous place steeped in history. Just down the road was the villa of legendary explorer Amerigo Vespucci, after whom America is named. And right next door to Vespucci's villa, within sight of Preston's house, was another, grimmer landmark.

Doug Preston: The scene of one of the most horrific killings in Italian history.

Stone Phillips: A lot of people would hear that there was a murder just up on that hill, and be a little spooked by it. Maybe -- maybe move, find another house.
Doug Preston: Well, I was a little spooked. Obviously my landlord never said anything about it. But the thing is that it interested me.

After all, he was a mystery writer. These murders had never been solved, and Preston soon learned that the killer had a name.

Stone Phillips: The Monster of Florence. Had you ever heard of it?
Doug Preston: I'd never heard of it … and I was really intrigued, the Monster of Florence. What a yoking together of two disparate words. I mean, you think of Florence, this beautiful renaissance city, the birth of civilization. And then the Monster of Florence. I found that very intriguing. I had to know more about it.

His research soon led him to a man named Mario Spezi, a well-known newspaper reporter in Florence.

Doug Preston: He's the local expert. Actually at the paper they called him the "monstrologer."
Stone Phillips: Because he knew so much about it--
Doug Preston: Because every killing -- he covered it. That's right.

The two men met, and struck up an instant friendship. Soon they were discussing writing a book together about the case. But there was something else, too.

Doug Preston: I saw the obsession in Mario when I first met him.

Obsession. It's a word that will come up again and again. But Spezi says it was pure chance that plunged him into the abyss. It was June, 1981 -- a Sunday -- when he got word of a double murder in the hills outside Florence.

It's about as lovely as you can imagine, with olive groves and wildflowers, along with a panoramic view of the city below. A perfect place for young couples to park and make love, which is exactly what the victims had been doing.

Spezi says the crime scene is still vivid in his mind, 25 years later. It was a scene worthy of Hannibal Lecter, but this was real. And here's a warning: the details are graphic, and disturbing.

Mario Spezi: What I remember here was the car and the boy was in the drive seat, and it looked like someone was sleeping.

Video: 'Monster' victim had 'eyes open wide'
But the young man, just 30 years old, was not sleeping in the driver's seat. He was dead, with a bullet in his head. Spezi didn't see the second victim, who was a woman, until a police officer pointed across a dirt road.

Stone Phillips: So, he didn't want to go. He told you where she was.
Mario Spezi: Right. He wanted not to see again.

And when he found her, Spezi understood why.

Mario Spezi: She was positioned like someone is looking to the sky, with the eyes opened wide.
Stone Phillips: She was on her back.
Mario Spezi: Yeah.

The woman, 21, had also been shot to death, then dragged into a field of wildflowers, her gold necklace between her lips. It was almost as if she'd been posed. And there was something else -- something ghastly.

According to author Doug Preston, “The medical examiner's report was quite horrific. He said that the mutilation had been performed with three very swift, powerful and expert cuts with a knife. Probably a scuba knife.”

Stone Phillips: Why a scuba knife?
Doug Preston: Well a scuba knife has a peculiar notches in it which most knives don't have.

The killer had left only one hard piece of evidence -- shell casings from a 22-caliber pistol. Police quickly tested them and identified the type of gun that fired them as a long-barrel Beretta, a common gun in Italy. But this particular gun was different.

Doug Preston: The firing pin of this gun left an unmistakable mark. Because it had a defect in it. That no other gun could leave. And so this became a very important clue.

The next day's paper carried Mario Spezi's story on the murder, and another reporter thought it sounded familiar. He remembered a double murder in 1974 -- seven years earlier.

That double murder also involved young lovers on a country road, but north of Florence -- 30 miles away from the recent crime. The young woman had been shot, then stabbed tentatively, with just the tip of the knife, dozens of times. A vine was inserted in her vagina. On the ground had been 22-caliber shells. Police, of course, read the story too.

Doug Preston: They immediately went back to the shells of that killing. And found that in fact, they were from the same gun.

The same defective firing pin marked both sets of shells.

Doug Preston: It was very shocking. Because it suddenly told the city of Florence: this isn't just an isolated, psychopathic killing. A serial killer is stalking the hills.

Here were two double murders, seven years apart. Four young lovers dead. One of the women sexually mutilated.

Mario Spezi, then a young newspaper reporter, wrote story after story about the case. He had heard of other famous serial killers dubbed "monsters" -- and he began to use that term as well. “We call him the Monster of Florence,” says Spezi.

Police desperately hunted the monster. In the process, they exposed some aspects of Italian life you don't read about in the tourist brochures.

Doug Preston: Most people live with their parents until they're married, and so making love in parked cars is a national pastime. The beautiful hills surrounding Florence on Friday and Saturday night were full of cars parked where kids were making out and making love.

All the love-making was an open secret among Florentines. But what police found next was not.

Investigating the terrible murders along lovers' lanes like this one in the beautiful Tuscan countryside, police uncovered a subculture that many Florentines found disturbing: these hills were not only being stalked by a killer, they were also swarming with peeping toms.

Police quickly zeroed in on one of these voyeurs.

Doug Preston: He lied to them at first. He lied about his movements that night. So they thought, "We've got our man." They arrested him.

But then, in October of 1981, with the suspect in jail, the monster struck.

Once again, the victims were young lovers in their 20's, shot to death on a lovers’ lane in the countryside. These were murders five and six.

The same Beretta .22 was the murder weapon. The young woman suffered the same post-mortem mutilation, performed with a notched knife.

This murder set up a pattern that would repeat itself. Police arrested a suspect, and the killer, almost as if taunting them, killed again.

Preston: And the police were humiliated and had to release him.

The monster struck again eight months later, in June 1982, about 10 miles south of Florence. A young couple had parked just off a busy road. This time, the young man apparently spotted the killer.

Stone Phillips: He saw the murderer coming?
Mario Spezi: Yes, because he tried to -- to start with the car. And -- the murderer to stop him, shoot the boy.

The young man managed to back the car across the road. But his rear wheels got stuck in a ditch. The car wouldn't move.

Doug Preston: The monster shot the two headlights out. And then he fired a shot that struck the boy in the middle of the forehead. And then when he went across the street and got into the car, he shot the boy a second time.
Stone Phillips: And he shot the girl?
Doug Preston: And he shot the girl when he got in the car.

But with the car stuck beside a busy road, the monster apparently did not feel he had enough time to perform his ritual mutilation of the woman. He fled, once again leaving no clues to his identity. These were murders seven and eight.

As the killings continued, the terror and paranoia ratcheted up. Florentines changed their daily routines. They never traveled alone, and they eyed each other suspiciously because the monster could be any one of them.

At one point a witness thought he had seen the killer. Police released a sketch.

The result was chaos.

Preston: There was a man who owned a pizzeria outside of Florence who looked just like him and was so harassed by his neighbors that he cut his throat, he committed suicide. There was a butcher who looked just like this fellow … a mob formed in front of his butcher shop and the police had to come and disperse the mob. There was a taxi driver who looked like this fellow … the people would scream and jump out of the taxi.

Doug Preston: About a double killing that took place in 1968.
Stone Phillips: All the way back to '68?
Doug Preston: All the way back to '68. And scrawled on this clipping was a sentence, you know, "take another look at this crime."

The spent shells from the '68 murder were still in the evidence room. Police tested them and were astonished to find that they matched the monster's gun.

Stone Phillips: So the gun -- so the same gun, the same bullets were used in this 1968 killing.
Doug Preston: Yes. And it was also the same m.o. -- a woman and a man who had been making love in a car who were killed in the act of making love.
Stone Phillips: So was it the monster?
Doug Preston: This is what they immediately thought. It must be the monster. But the strangest thing was that this crime had been solved. The killer had been found.

The year was 1968. The crime was a double murder. The victims were a man and a woman making love in a parked car. The gun was the very same .22 caliber Beretta that the Monster of Florence was using to terrorize the Tuscan countryside. In their frantic hunt for the monster, police re-opened what had seemed at the time like an open-and-shut case.

Doug Preston: The killer was the husband of the woman who was having an affair with somebody.

The killer's name was Stefano Mele, and all during the ‘70s and early ‘80s when the monster killings took place he was either in prison or a halfway house.

Doug Preston: There was no way this guy could be the monster.
Stone Phillips: And yet, somehow, the same gun was used in a '68 killing and these monster killings.
Doug Preston: That's right. Exactly.

How could the gun have passed from Stefano Mele to the Monster of Florence? Italian reporter Mario Spezi got a clue when he managed to interview Mele at the halfway house where he was being held.

Spezi: This is very important. He said, "they will kill again." "They," not, "he."

In the words of author Doug Preston, “Who is 'they'? And that's when Spezi realized that this man had not acted alone. He'd had at least two accomplices.”

There were so many secrets buried in the Tuscan hills, and another was about to be unearthed. Police slowly realized that the 1968 murder was far more than the act of a jealous husband.

Doug Preston: It was actually a group of Sardinians who had settled in Tuscany. And it appeared to be a clan killing in which the husband was the fall guy.

The group from the island of Sardinia was known to be insular and violent. Its leader, Salvatore Vinci, had a bizarre relationship with Stefano Mele and his wife.

Doug Preston: It turned out that they'd been involved in absolutely kinky and depraved group sex encounters. Where this woman was the center of attention. They called her the queen bee.

And, Preston claims, Vinci was enraged when the queen bee -- who had slept with all his Sardinian countrymen -- started an affair with an outsider, a Sicilian.

Preston: In this case, the monster had done more than just remove the woman's vagina. He had also cut off and taken away her left breast.

These were murders 11 and 12. There was outrage across Italy. And now police were the focus of it.

Doug Preston: Because again and again the police had been arresting people. And again and again, the monster had been killing people when those suspects were in custody.

In September 1985, the monster committed a crime that Hannibal Lecter would find hard to top.

The young lovers this time were tourists from France who were camping in the hills south of Florence. As they made love in their tent, the monster shot the woman in the face, killing her instantly. The man was shot several times, but somehow managed to burst from the tent.

Doug Preston: And here's someone who's running for his life … and yet the killer was actually able to catch him, reached up behind him, cut his throat.

The killer then returned to the tent and mutilated the dead woman in his new, more terrible way. But he still wasn't done.

Doug Preston: One of the prosecutors in the case -- a woman named Silvia della Monica -- received in the mail a letter, addressed like a ransom note [with] letters cut out of a newspaper ... And inside was one item. It was the nipple from the victim.

Italian authorities sought help from outside the country. They asked the FBI's behavioral science unit to formulate a psychological profile of the monster.

The FBI report described a lone killer
who was sexually impotent, acting out a "ritualized anger" toward women. It was someone who felt he could only possess a woman by murdering her and mutilating her body.

Italian investigators questioned hundreds of men, but they made no progress until they received an anonymous letter about a farmer who lived just outside Florence.

Preston: Pietro Pacciani. He was known to be very violent. He beat his children. He beat his wife. People were frightened of him.

He was certainly a monster. But was he the monster?

In November 1994, Pietro Pacciani, after being fingered by an anonymous tip, went on trial.

He was accused of being Italy's most notorious serial killer. It had been almost nine years since the murders had mysteriously stopped. Now all of Italy was focused on one question:

Was Pacciani the Monster of Florence?

Preston: He was a drunken peasant, who, as a young man in 1951, had caught his fiancé being seduced by a traveling salesman. He had killed the traveling salesman, stomped his head in and then raped his fiancé next to his body.

Prosecutors claimed the murderous Pacciani had said something that seemed to link him to the monster killings decades later.

Doug Preston: When he saw his fiancé uncover her left breast, that's when he'd gone crazy.
Stone Phillips: The left breast.
Doug Preston: Very important clue.

Important, because in his last two killings, the monster had cut off the left breast of his female victims. And why did the monster killings suddenly stop in the mid-1980's? Pacciani's history offered a sordid explanation for that, too.

Police found another set of clues when they searched Pacciani's house. First, an erotic print of a woman, her breast exposed, with what looks like a flower between her lips. Second, a fine art print.

Preston: They found a reproduction of that famous painting by Botticelli, "The Primavera."

Uffizi Galllery

Botticelli's "Primavera"

The original hangs in Florence's Uffizi Gallery. On the right you see a nymph being stalked by Hermes, the Greek god who escorts the dead to the underworld. And then, when you look closer, as author Doug Preston notes, you see “flowers and vines are spilling out of her mouth. She's kind of vomiting flowers and vines.”

Remember the 1981 murder -- where the mutilated young woman was found in a field of wildflowers, her necklace draped across her lips? For the prosecutor, that image plus the nude and the Botticelli added up to evidence against Pacciani.

Prosecutors argued he staged the '81 crime scene to feed some strange obsession.

Stone Phillips: How does a Renaissance painting figure into a murder mystery?
Doug Preston: Well, this is Italy. Where history lives on in the present. And you know the idea that a renaissance painting is a clue to a modern crime is very sexy and appealing.

Pacciani's trial was attended by families of the monster's young victims and became a media sensation.

Preston: They had Pacciani's daughters testifying about how he had raped them. The testimony from the murder that he committed in 1951 was horrifying … All this just made Pacciani look like a monster.
Stone: And when it was over?
Preston: He was convicted. As you might expect.

But Italian journalist Mario Spezi never believed Pacciani was the monster. For one thing, the gun and the knife that connected all the murders had never been found and they were never linked to Pacciani.

What's more, Spezi had been to every crime scene, and he knew the killer had to be smart, fast, and skillful -- nothing like the dim, drunk, overweight Pacciani.

Spezi also saw a glaring contradiction. Pacciani was a sex criminal. He was a convicted rapist. For the monster, however, mutilation seemed to take the place of sex.

Stone Phillips: Was there any evidence that the killer had sexually assaulted the victim?
Mario Spezi: Never, never. He never had sexual--
Stone Phillips: Activity with the-- with the--
Mario Spezi: --activity with the victims. Never.

Spezi had also seen that FBI profile of the monster and thought it just didn't match Pacciani at all.

The profile said the killer was probably sexually dysfunctional, which Pacciani was not.

The profile said the killer probably lived alone or with an older relative, yet Pacciani had a wife and children.

It also said the killer was probably in his 20s at the time of the first murder. Pacciani had been almost 50.

Mario Spezi: You know, from a theory of the Botticelli paintings and the way of working FBI, I prefer FBI.

Video: Why did 'Monster' go to lovers lanes?
In 1996, Pacciani's conviction was overturned and he was released. But then the monster case, already as intricate as the Duomo, got even more complicated. A new witness came forward to say that he was involved in the killings.

Preston: He said, "We were working for somebody else … who needed body parts." Well, immediately the question arose, what was the purpose of the body parts? And the question was quickly answered, "For satanic rituals. For black masses. For offerings to the devil."

Incredible as it seems, even though the witness admitted he had never met the Satanists – and even though there were signs he was mentally unstable -- police jumped on the new theory that a satanic sect was behind the monster killings.

Michele Giuttari: In this story of the Monster of Florence, there are elements that point to the theme of satanism.

Michele Giuttari, a tough talking, cigar-chomping veteran of the Florence police department, says the evidence includes stone circles found not far from one crime scene.

Giuttari: Inside one of these circles were found two roses and a wooden cross stuck upside down in the ground. This is clearly a satanic symbol.

Spezi developed his own theory based on something he says he was told by a high ranking member of the Carabinieri, the Italian federal police.

Spezi: They tell me to a journalist who's writing about the monster, they told me this new story. It was very interesting.

The Carabinieri had withdrawn from the case years before, reportedly outraged at the way it was being managed by local investigators.

Doug Preston: But obsession is obsession. They continued a secret investigation into the Sardinian connection to see if they could figure out who the Monster of Florence was.

The unofficial investigation had led to a suspect. He was the son of one of the Sardinians involved in the '68 murder so he could have gotten hold of the gun, which was the key to the whole mystery.

Spezi: This is the real, real problem, of the case of the Monster of Florence.
Stone Phillips: Who has the gun and how they got it.
Mario Spezi: Yeah.

Spezi interviewed people who knew the suspect. They told him the man was a crack shot and an expert with a knife. He had lived in another part of Italy during the late ‘70s when there was that mysterious gap in the monster killings.

Spezi began to compare the new suspect against the FBI profile of the monster and found key similarities.

The FBI said the monster probably picked murder locations he knew well. Spezi found the suspect had lived near all the murder sites.

The FBI said the monster was probably sexually dysfunctional. Spezi found out that at the height of the killing spree, the suspect had a marriage annulled for inability to conceive children, which Spezi believes was code for impotence.

Remember the first monster killing in 1974 with those tentative stab wounds? The new suspect would have been just 15 years old at the time. Perhaps he was still uncertain what his murderous ritual would be.

While inspector Giuttari chased satanists, Spezi spent years looking closer and closer at the Carabinieri's suspect. By the time he met Preston in 2000, he had a convincing case.

Doug Preston: The more we looked at it, the more we eliminated other possible suspects, the more it seemed likely that he was the person. That he was the Monster of Florence.

They began to refer to him by the pseudonym “Carlo” and agreed he would be the focus of their book. They decided they had to talk to him.

Doug Preston: We went to his house at 9:30 at night. Rang his doorbell. We got buzzed up. No problem.

After two decades on the trail, was Spezi -- along with Preston -- about to confront the Monster of Florence?

The Monster of Florence murdered 14 young lovers and terrorized a city. He eluded capture for 30 years. Now, authors Mario Spezi and Douglas Preston had arrived unannounced at the home of "Carlo," a truck driver they suspected might be Italy's most notorious serial killer.

Doug Preston: He invited us in with great charm and welcome. He was a very charismatic individual. With big rippling muscles. And tattoos. And scars on his body.
Stone Phillips: And how old?
Doug Preston: Mid-40s. He seated us at his kitchen table. He offered us a glass of a special type of Sardinian liquor.
Mario Spezi: He was a very intelligent man. And he joked with us.
Preston: And we proceeded to ask him questions. Very gently at first. General questions. And finally the questions got more and more pointed.

He denied ever having the monster's gun. But he did say he owned a knife.

Doug Preston: A scuba knife. That that was his knife of choice.
Stone Phillips: The same kind of knife with a notch that had been used in the killings?
Doug Preston: In the killings.

Then Spezi asked the biggest question of all.

Mario Spezi: "So you are not the Monster of Florence?"
Stone Phillips: You asked him outright?
Mario Spezi: I asked him directly. And -- he [said] "I'm sorry, I can't let you do this scoop.”
Stone Phillips: "I can't let you do this scoop."
Doug Preston: And then he said something very vulgar in Italian.
Stone Phillips: The gist of it being what?
Doug Preston: Well, the gist of it was “I like my women living when I have sex with them.”

Mario Spezi: He said, "Ah, Spezi, I am forget something. I never joke."
Doug Preston: “And I never kid around."
Stone Phillips: What do you take that to mean?
Doug Preston: Well, it was a threat. In Italian it's even more of a threat than it is in English.
Stone Phillips: As you left his home after speaking to him, what did you think?
Mario Spezi: Well. Me and Doug we were silent. We enter our car. And then almost the same time, we say it's him.

If it was him -- if Carlo was the monster -- it was a stunning moment. It would make a great ending to their book on the monster case. But at the same time, another author was also at work on that other theory of the crimes.

Giuttari: I believe I have done my duty, seriously and for many years. I wanted to make the recent developments official in my book so that this story is not forgotten.

Florence police inspector Michele Giuttari was writing his own book, and who could blame him? Books on the monster case are big sellers in Italy. Giuttari thought he had compelling evidence that a satanic cult was behind the monster killings, such as that oddly shaped stone found at one of the murder scenes.

Giuttari: [in Italian] A uniquely shaped rock was found in the form of a truncated pyramid, to which the experts on satanism granted importance.

Florence was intrigued. Mario Spezi was amused.

Spezi: I called some friends of mine. And in an afternoon, I find -- I found seven.
Stone Phillips: Seven of these?
Mario Spezi: Yes, it's a common object.

What was this strangely carved stone?

Preston: An antique Tuscan doorstop. You can find them in antique stores all over Tuscany.

The foundation of the satanic cult theory was a doorstop. Spezi soon published his findings in the newspaper.

It was a classic spat between two writers, except that one of the writers was also a cop.

Preston: The police arrived at Spezi's apartment. Six o'clock in the morning. Turned the place upside-down. And -- then behind Spezi's door, they found the doorstop. Later in the report they made they said that now they had evidence that connected Spezi directly to the scene of one of the crimes and to the satanic sect.

It seemed incredible. But it was no joke.

Doug Preston: Here's a guy who knows everything. He's followed the case obsessively.
Stone Phillips: And he had the hexagonal stone.
Doug Preston: And he had the hexagonal stone.

Mario Spezi, a reporter, also became a suspect.

Stone Phillips: So you are under investigation?
Mario Spezi: Yes.
Stone Phillips: For murder.
Mario Spezi: For murder.

Just when you think this case couldn't get any stranger, there’s another twist. An ex-convict came to Spezi, and in return for a few euros he gave him a white hot tip.

The source claimed that Carlo had taken him to one of his hideaways on the grounds of this centuries-old villa, and what he'd seen inside might crack the monster case once and for all.

Mario Spezi: He tell me that he saw in this house the gun and in a little armoire, six metallic boxes.

The gun, and in an armoire, six boxes – which matches the number of women who'd been mutilated by the monster. It was tantalizing, and if true it was proof that Carlo was the killer.

Stone Phillips: Did you believe it?
Mario Spezi: Well, I -- yes.
Doug Preston: Well, Spezi just about had a heart attack. So he's telling me this and I said, "Mario, this sounds too good to be true."

Preston was intrigued enough, however, that he asked Spezi to take him to the villa.

Stone Phillips: And so Mario's thinking, finally this is the place.
Preston: Exactly.

Preston says they didn't get a chance to investigate.

Preston: Well, we did a very quick little walk around but it was pouring rain and we didn't do anything else. We were there for no more than 10 to 15 minutes.

They found no answers that day, just more trouble than they ever imagined possible.

Preston: I was thinking towards the end of this thing, I'm never going to see my wife and children again.

In the spring of 2006, Douglas Preston and Mario Spezi's book on the Monster of Florence case was published in Italy. The authors described their lone suspect, whom they called by a pseudonym, "Carlo," and they ridiculed the work of police inspector Michele Giuttari, whose own book claimed a satanic cult was responsible for the murders.

Preston: They spin these outrageous theories. Satanic sects … body parts used in bizarre rituals. And then they start looking for the evidence to support the theory.

Giuttari claimed it was Spezi and Preston who were spinning a fairy tale.

Giuttari: If he writes a novel and then says it's the truth it's not right, it's presumptuous. He isn't telling the truth.

The war of the writers was about to escalate.

Acting on a tip, Spezi and Preston had gone to an old villa searching for evidence against Carlo. They found nothing. But soon afterward, Preston's cell phone rang. A judge overseeing the monster case wanted to talk to him immediately. Preston went to his office.

Stone Phillips: What did they want to know?
Doug Preston: They asked, "Well, why did you go to the villa? What did you do there? How long did you spend there? And, you know, my Italian is not perfect, I started stumbling and stammering, which I do in Italian. And I -- it suddenly occurred to me, my God, I sound like I'm lying to them.

Preston: The judge nodded to the stenographer, she pressed a button on her computer, and here's my voice. And here I am talking to Mario Spezi.

Giuttari and his men had been tapping Spezi's phone. They'd even broken into his car, planting bugs there, too. They had recorded him and Preston discussing the case. And on tape Spezi said something that struck the judge as highly suspicious --

Stone Phillips: “We did it all.”
Mario Spezi: Yeah-- we did it. We did it. Yes.

Spezi says what he meant was that he had turned all his information about Carlo over to police. But the judge interrogating Preston had a different interpretation.

Preston: He said, "You went to the villa to plant evidence to incriminate an innocent man of being the Monster of Florence to deflect suspicion from Mario Spezi himself, who as you know is being investigated for murder.
Stone Phillips: This is like out of a crime novel.
Doug Preston: Yeah, it is. And he said “You're an accessory to murder if you don't tell us what you know.” At this point, I thought they were going to put me in handcuffs and take me into a cell or something.

The judge told him he had another option: to get out of the country.

Preston: I left Italy the next day.

But Mario Spezi had nowhere to go.

Stone Phillips: They arrested you?
Mario Spezi: Yeah, and I was six days -- six days in a cell without seeing anyone.
Stone Phillips: In solitary confinement?
Mario Spezi: Yeah.

And the man who put him there was his literary rival inspector Michele Giuttari. Giuttari now says he does not suspect Spezi of murder, but he does think he obstructed justice.

Stone Phillips: Why would Preston and Spezi plant evidence?
Giuttari: Probably because this would have been proof of the Sardinian connection and therefore all the work that the police and the prosecutor's office had done was mistaken. It's also possible that he had his book in mind.

Inspector Giuttari told us he had proof that Spezi and Preston were dead wrong about "Carlo," the man they suspected of being the monster.

Giuttari: Evidently, Mr. Preston did not do the least bit of fact checking … In 1983, when the two young Germans were killed, this person was in prison for another crime unrelated to the monster crimes.

Since Giuttari was so sure that Carlo was not the monster, we asked if he'd set up a meeting for us. That night, at Giuttari's office, it happened.

After conferring with Giuttari for nearly an hour, Carlo agreed to speak to us, but not on camera.

Standing face-to-face, at times uncomfortably close, Carlo answered our questions with a smile that barely concealed his contempt.

We asked if he was good with a knife. He said no, although he does own a scuba knife. He's a diver.

We asked if he was a good shot, and he said he'd never fired a gun -- not even a toy.

We asked about his prison record. He said he was sure he'd been in prison during one of the monster killings, but he just couldn't remember which one.

We later checked his record and found that in fact Carlo had never been in jail during any of the monster killings. He and Giuttari were either mistaken or lying about that.

Carlo did have a criminal record. The man who claimed he'd never fired even a toy gun had been arrested for illegal possession of firearms.

We asked Carlo flat out, "Are you the Monster of Florence?" He locked eyes, gripped my hand, and said one word.

When we asked if there was anything more he wanted to tell us, he said what really made him angry was Spezi and Preston's suggestion that he was sexually impotent. His words, and I quote, were "If Spezi's wife were younger and prettier, I'd show them who isn't impotent -- I'd show you right here, right now, on this table."

So here we are, more than 20 years after the end of a killing spree that left 14 young people dead.

The murder weapon has never been found.

There's no definitive proof of a Sardinian connection, let alone proof of a satanic cult.

The guy who tipped Spezi about the villa and the iron boxes? He was a con man, out to make a few bucks.

The endless investigation has now turned into a monster itself, eating more of its own. The judge who interrogated Doug Preston is under investigation for abuse of his office, as is inspector Giuttari, who told us Carlo was in jail when he really wasn't.

Giuttari is also serving a suspended sentence for making false statements in an unrelated case.

What about Mario Spezi? A high court in Rome finally cleared him of criminal wrongdoing. He says the book about "Carlo" is the last thing he'll ever write on the monster case.

Stone Phillips: Do you believe he's the murderer?
Mario Spezi: I can't prove. But I believe.

Does it even matter anymore?

Pia Rontini was just 18 in 1984, when the monster killed her and her boyfriend. Pia's mother told us she stopped wondering who the monster is long ago because knowing won't bring Pia back.

Doug Preston, however, says he has to know.

Doug Preston: Why do people become obsessed? You just think you're so close. I think the next bit of information that I turn up is going to strike gold, it's going to illuminate the truth … the truth is lurking somewhere in the world. All we have to do is find it.

The monster may never be caught, but according to one famous Florentine, he'll never escape, either.

In his epic poem "The Inferno," renowned Italian poet Dante Alighieri detailed the punishments he said awaited each sinner in hell. The murderers, he wrote, are boiled in blood.

Dateline wires the home of a volunteer, Jenny, from top to bottom with hidden cameras. Then she called repairmen to her house for a simple problem we created as a test with Jenny's pool. NBC's Chris Hansen reports.